1. Field of the Invention
The present invention relates to an ink-droplet ejecting apparatus of an inkjet type that drives an actuator to produce a pressure wave or vibration in a pressure chamber and thereby eject a droplet of ink from a nozzle.
2. Discussion of Related Art
There has been known an ink-droplet ejecting device including an ink ejection nozzle, a pressure chamber (i.e., an ink chamber) that is provided in rear of the nozzle and is filled with ink, an actuator that changes a volume of the pressure chamber, and a control device that drives the actuator to produce a pressure wave or vibration in the pressure chamber and thereby eject a droplet of the ink from the nozzle.
For example, Patent Document 1 (Japanese Patent Application Publication No. P2003-231263A or its corresponding U.S. Patent Application Publications Nos. 2003/0146956A1 and 2004/0246315A1 or Patent Document 2 (Japanese Patent Application Publication No. P2003-145750A or its corresponding U.S. Pat. No. 6,523,923) discloses a piezoelectric actuator that utilizes a piezoelectric effect that when a drive voltage is applied to a piezoelectric element, an elastic deformation of the element occurs. In the disclosed piezoelectric actuator, first piezoelectric sheets (i.e., piezoelectric-ceramics sheets) on each of which an individual-electrode layer including a plurality of individual electrodes is formed, and second piezoelectric sheets on each of which a common-electrode layer constituting a common electrode is formed are stacked alternately on each other. A high electric voltage is applied to all pairs of individual electrode and common electrode so as to polarize, in advance, respective portions of the piezoelectric sheets that are sandwiched by the respective pairs of individual electrode and common electrode and thereby form the sandwiched portions into active portions. When a drive pulse having a low electric voltage is applied to an arbitrary pair of individual electrode and common electrode, according to a print command, a corresponding one of the active portions is elastically deformed in the direction of stacking of the piezoelectric sheets, and accordingly a volume of a corresponding one of pressure chambers is changed.
Patent Document 3 (Japanese Patent Application Publication No. P2000-052561A or its corresponding U.S. Pat. No. 6,412,923) discloses another piezoelectric actuator in which actuator walls provided on either side of a pressure chamber (an ink chamber) are deformed in a piezoelectric shear mode and accordingly a volume of the pressure chamber is changed.
In each of the above-described piezoelectric actuators, if a pressure wave is applied to, and canceled from, the pressure chamber at a timing corresponding to a period at which each pressure wave propagates one way in the pressure chamber in a longitudinal direction thereof that is, if a pressure wave is applied at a timing when the pressure of the ink in the pressure chamber increases and a pressure wave is canceled at a timing when the pressure of the ink in the pressure chamber decreases, the pressure wave is amplified so that the second ink droplet next to the first ink droplet is ejected more strongly than the first ink droplet, and so on. Thus, the ink can be ejected with improved pressure efficiency.
In particular, in the piezoelectric actuator disclosed by Patent Document 2, a plurality of drive pulse signals are applied according to one print command, so that a plurality of ink droplets are ejected from one nozzle and one dot is formed with a large amount of ink on a recording sheet as a sort of recording medium. Thus, an image having a high density can be printed on the recording sheet.
Meanwhile, in the piezoelectric actuator disclosed by Patent Document 3, two ink ejecting actions occur to eject two ink droplets and thereby form one dot, in such a manner that a first ejection pulse signal is applied to eject a first ink droplet, and then a first non-ejection pulse signal is applied to cancel a pressure wave produced by the first ink ejecting action Subsequently, in a state in which the pressure in a pressure chamber has been sufficiently stabilized, a second ejection pulse signal is applied to eject a second ink droplet, and then a second non-ejection pulse signal is applied to cancel a pressure wave produced by the second ink ejecting action. Thus, it is said that even if a frequency at which the actuator is driven may be more or less changed, an image can be printed with good quality.
However, recently, inkjet image recording devices have been required to record images at higher speeds. That is, an actuator that can be driven at higher frequencies to eject ink droplets is demanded.
In addition, inkjet image recording devices have been required to record images with a printing quality comparable to that of photographic images, in such a manner that the recorded images have many colors and many halftones, that is, an amount of an ink droplet corresponding to one pressure wave is small and a total number of dots formed in unit area on a recording sheet is great. That is, an actuator that can be used with a nozzle having a small diameter is demanded.
In particular, in the case where ink ejection commands are discontinued from each other by a non-ejection command, that is, a print period corresponding to an ink ejection command is followed by another print period corresponding to a non ejection command, if an ink droplet is erroneously ejected toward a position on a recording sheet where no ink should be ejected, a quality of an image formed on the recording sheet is significantly lowered. Hence, Patent Document 3 teaches applying, after applying an ejection pulse signal, a cancel pulse signal so as not to eject an ink droplet toward a position where no ink should be ejected.
Moreover, when an environmental temperature around an ink-droplet ejecting device, that is, a temperature of ink increases, a viscosity of the ink decreases. In other words, when the environmental temperature decreases, the viscosity of the ink increases. Thus, in order to prevent ejection of an excessively large, or small, amount of ink (i.e., an excessively large or small volume of each ink droplet), it is needed to control accurately a timing when a cancel pulse signal is applied after application of an ejection pulse signal and/or a pulse length of the cancel pulse signal.